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Old 9th November 2008, 07:04 AM   #11
Tino is offline Tino  Canada
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Thanks everyone for the replies. I feel much more comfortable doing the swap. Indeed this Threshold is a small wattage model (SA3 - 50wpc) as many of you gathered. I posted in the pass labs area just incase if it shared any similarities with Mr. Nelsons more current stuff
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Old 9th November 2008, 07:07 AM   #12
Tino is offline Tino  Canada
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Hi Mooly,

Actually thats why I'm replacing the caps. I figured maybe the hum might be due to the caps having degraded over time. I dont know any way of measuring ripple easily since I dont have a scope or anything - unless there is some way of figuring it out with a standard multimeter...

PS. The hum is very slight in one channel and I do notice inside the amp, the tranformer is not quite centered...infact the hum IS on the side that the transformer is closer to so I wonder if its just the left channel being closer to the trasformer...Hmmm[

QUOTE]Originally posted by Mooly
Hi,
I would be surprised if the cause of the hum are the caps. Is that the main reason you want to change them. Have you measured the ripple on them.
The killer of electroylitics is mainly heat. The bridge rectifier is in a strange place bad heat wise, good physically. Does it get hot ?
Most causes of hum are down to wiring errors and induced hum from the magnetic field of the amp.
[/QUOTE]
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Old 9th November 2008, 07:29 AM   #13
Mooly is offline Mooly  United Kingdom
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Is it easy to lift the transformer and temporarily extend the wires. Toroids can be usefully rotated to find a null point ( sometimes )
Have you run the amp up with nothing but speakers connected and shorting plugs fited on the inputs. Nothing else connected.
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Old 10th November 2008, 04:42 AM   #14
Tino is offline Tino  Canada
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Hi Mooly, I think the transfomer is mounted from underneath via a bolt so maybe its movable. I should check into that. I have run the amp with only speakers connected but no rca cables attached - however I did not use any shorting plugs.

I found a document that said how to make some but I dont recall the link now. All I need is the resistor value I guess. I have a whole bunch of 1K 1/4 watt resistors but I dont know if 1K is ok to use or not. Time for google I guess

I read some people troubleshooting ground issues by using putting on an adapter plug that is missing the ground pin...but somehow this makes me paranoid so I dont want to do that.

The hum is very light and not audible even from a foot away. I wonder if i should leave it alone. I did take the amp to a local stereo shop and the tech said he couldnt find anything on the output that would be of any concern...he felt that everything was within spec. Still if I can get the amp to be more silent on that one channel, it would be nice.

Quote:
Originally posted by Mooly
Is it easy to lift the transformer and temporarily extend the wires. Toroids can be usefully rotated to find a null point ( sometimes )
Have you run the amp up with nothing but speakers connected and shorting plugs fited on the inputs. Nothing else connected.
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Old 10th November 2008, 07:53 AM   #15
Mooly is offline Mooly  United Kingdom
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Hi,
You don't need a resistor, just short the inputs directly. I take it they are phono ( RCA ) sockets. Just get two old plugs and short them out, center to outer.
As to lifting grounds, if the amp hums as I say, with just the amp speakers and inputs shorted, and NO other items connected it's not an external ground loop.
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Old 10th November 2008, 04:15 PM   #16
Tino is offline Tino  Canada
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Hi Mooly, are you sure shorting the inputs are ok because I thought I remembered reading long ago that the threshold amps dont like to have their inputs shorted (like with no resistance). Anyway I will have to check into that.

Thanks
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Old 10th November 2008, 05:05 PM   #17
Mooly is offline Mooly  United Kingdom
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Never heard of an amp that you couldn't short the inputs on. If you are unhappy use a 1 k or similar.
A passive preamp would short the inputs, wiper of volume pot to ground when on minimum volume
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Old 10th November 2008, 05:32 PM   #18
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The SA/3 has a matched JFET differential input, like the bigger SA/1 and SA/2 models from the same time frame.
Shorting the input is no problem.

(the transformers and heatsinks of the 80s series were also identical, close to modular building block range, pretty neat. The SA/2 is an SA/3 in monaural disguise, with standard 80V caps it's a piece of cake to convert an SA/3 to an SA/2 )
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Old 10th November 2008, 11:13 PM   #19
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Quote:
Originally posted by Zero Cool
I just don't want to see anyone get hurt by an exploding cap or something. I have had it happen to me and its not fun to have to dig shrapnel out of your face! So please double check and be careful.
Eye protection and ear protection are essential.

Eye protection is obvious, but if you have ever felt the supersonic
whistle of a cap venting you would use ear plugs as well.

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Old 11th November 2008, 04:38 AM   #20
Tino is offline Tino  Canada
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Hi Nelson. Let me say that I love this amp more than any other piece of audio gear I own. I've taken this amp quite a few times to stores and compared it with much more current and sometimes quite expensive amps and its not even close. The smoothness, huge soundstage and control. I love it. You are a wizard with electronics.

Any tips for me replacing the caps? If it were you, would you go with 37,000UF 75V caps or 30,000UF 100V caps? Also not sure if the very slight hum on the left channel could be cap related or maybe just a slight noise picked up by the transformer being closer inside the chassis to the left channel? Anyway its not noticable unless my ear is very close to the woofer so it might even be within spec. Regardless, this thing sounds fantastic regardless of its age.

One day I will have my dream of buying a mint condition Aleph 5. :-)
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